It is known to coat the surfaces of synthetic-resin bodies, for example acrylonitrile-butadiene-styrene (ABS) terpolymers with metals by a process which has been termed currentless, electroless or chemical plating. Typical among the processes provided for this purpose is that described in German Pat. No. 1,197,720 in which a palladium salt in finely divided colloidal form, together with a tin hydrosol, is brought into contact with the synthetic-resin body surface. This treatment applies a metal-precipitating catalyzing metal salt to the surface of the body, which surface may be chemically roughened, so that the body can then be brought into contact with a metal-coating bath to deposit the metal upon the body.
In the system described in this patent, however, the colloidal system requires a high hydrochloric acid concentration and the pH must be maintained well below 1 to prevent the colloidal palladium metal from precipitating from the liquid phase.
To remove the high acidity of the surface, the body must be thoroughly rinsed before chemical plating with the metal to be deposited. The rinsing water is thereby highly contaminated and constitutes an environmental pollution hazard. When the rinsing step is omitted, however, problems are encountered with the subsequent chemical plating. For example, ammonium chloride mist is produced when the chemical plating step makes use of conventional ammoniacal nickel baths. The colloid is also oxidation sensitive and tends to become unstable since the solution no longer contains their divalent tin ions.
The conventional process thus requires the removal of the greater part of the catalytically effective palladium metal from the synthetic-resin body surface together with the hydrolyzed tin sol in a special operating step and in the absence of this step results in a poor product. The step itself, however, produces a contaminated waste water whose disposal constitutes a problem.
Between the initial activation (catalyst-treatment step) annd the subsequent metal coating treatment it is necessary to provide a so-called acceleration treatment in a separate bath.
It has already been suggested to eliminate the acceleration treatment in copper-coating systems since the high alkalinity of a chemical plating copper bath solubilizes the tin protective coating without more. However, the elimination of the acceleration step renders the copper precipitation slow and creates problems with bubbling and poor adhesion in the event the metallized surface of the synthetic-resin body is to be galvanically plated subsequently.
Another disadvantage of this system (in which the acceleration step is eliminated) resides in that significant quantities of metallic palladium are carried into the copper bath so that the stability of this bath for chemical plating is reduced. Currentless copper plating has been found to be practical only in the printed-circuit arts while currentless metallization of other synthetic-resin surfaces for decorative effects have hitherto exclusively operated with chemical plating from nickel baths. Since the usual nickel chemical plating baths do not have either the acidity or the alkalinity which is necessary to remove the protective tin colloid, the acceleration step has been essential.
Thus for all practical purposes with conventional chemical plating nickel baths, the activation step (following the chemical roughening of the body surface) has been subdivided into two partial process steps, the first being the treatment with the catalyst in the colloid and the second being an acceleration step to enable chemical metal plating in a short period, preferably with a maximum of several minutes, i.e. a period preferably between 0.2 and 2 minutes, with uniform metal coating.
In the aforementioned German patent it has also been proposed to operate with alkali solutions in which the protective colloid is gelatin, albumen or tannic acid. In this system, of course, a subsequent acceleration treatment can be eliminated since these protective colloids are soluble in water. However, metallization of the synthetic-resin surface is not nearly as good with treatment from an alklali medium as treatment from acidic solutions which contain tin acid sols. Also with the alkali treatment, it is necessary to use long treatment times to obtain a uniform and effective coating.
In other conventional processes it is possible to eliminate the acceleration treatment and the disadvantages thereof, but these systems have other problems which also require two-stage treatments.
Thus it is known to chemically plate synthetic-resin with copper by chemically roughening the synthetic-resin surface in a chromic-sulfuric acid solution, sensitizing the surface in a tin (II)-chloride/hydrochloric acid solution activating the surface with ammoniacal silver nitrate, etc. Sensitization and activation are here the two process steps prior to chemical plating.
It is also known in a similar system to substitute a hydrochloric palladium solution or a palladium complex solution for the ammoniacal silver solution. In this sequence as well the sensitization step is reductive to the palladium compounds so that palladium nuclei coat the surface of the body and there serve to catalytically promote the deposition of copper or nickel for the copper or nickel chemical plating baths.
Other systems using the combination of sensitization and activation with tin chloride solutions or other reducing agents have also been proposed without eliminating the need for two-step treatment prior to chemical plating.
Thus all of the processes described above, to the extent they are practical, require a two-stage chemical treatment of the synthetic-resin surface between the chemical roughening step and the chemical plating from the metal bath.